A gas turbine engine commonly includes a fan section, a compressor, at least one combustor, and a turbine. The compressor and turbine each include a number of rows of blades attached to a rotating cylinder. In operation, the air is pressurized in a compressor and is then directed toward the combustor. Fuel is continuously injected into the combustor together with the compressed air. The mixture of fuel and air is ignited to create combustion gases that enter the turbine, which is rotatably driven as the high temperature, high pressure combustion gases expand in passing over the blades forming the turbine. Since the turbine is connected to the compressor via a shaft, the combustion gases that drive the turbine also drive the compressor, thereby restarting the ignition and combustion cycle.
Since the gas turbine engine operates at high temperatures, certain components of the gas turbine engine, such as linear flowpath liners, the turbine, combustor and augmentor, are directly exposed to hot combustion gases, the temperatures of which sometimes exceed the melting temperature of the materials used in the engine components in contact with these hot gases. To prevent damage to the components, solutions are needed to shield the components from excessive heat.
One common solution is to protect the affected surfaces of the components with a coating system, for example, an aluminide coating or a thermal barrier coating (TBC) which typically includes a metallic bond coat and a layer of ceramic deposited on the bond coat layer. A typical metallic bond coat includes, for example, MCrAlY, wherein M is Ni, Co, Fe or mixtures thereof. The bond coat provides oxidation and corrosion resistance, as well as accommodates residual stresses which might develop in the coating system. A commonly applied TBC material is yttria stabilized zirconia (YSZ) which exhibits resistance to thermal shock and thermal fatigue even at 1150° C. (2102° F.). Methods, such as air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) process such as electron beam physical vapor deposition (EBPVD) are typically used to deposit the ceramic layer on the bond coat.
In addition to applying a TBC system to the surface of the affected components, internal cooling of selected engine components, such as turbine blades, nozzles, and liners is employed to further protect the underlying component substrates. To accomplish effective cooling, a complex cooling scheme is usually installed which forces bleed air to exit from cooling holes on the flowpath surface and form a suitable film of cooling air over the flowpath surface.
When a TBC is damaged during operation or when a new design of TBC needs to be installed, the old TBC often needs be removed before the new TBC is applied. However, the presence of open cooling holes on the exposed surface of the engine component after the removal of the old TBC layer poses a significant problem for the application of a new, high quality TBC layer. Specifically, a non-uniform (or uncompacted) surface coating susceptible to spallation frequently results when a new TBC layer is directly applied over pre-existing open cooling holes remaining after removal of the old TBC layer. In particular, since the new cooling holes are drilled to meter a specific quantity of cooling air on the engine component after a new TBC layer is applied, any subsequent coating spallation may lead to opening of the pre-existing cooling holes and cause an increase in cooling air flow on the component, as cooling air flow is metered by the size and quantity of the cooling holes. The increase in cooling air flow on the component may subsequently starve other downstream components of cooling air causing the downstream components to suffer from structural damage associated with operating at higher than designed temperatures.
It is known that a typical braze repair process may be used to obstruct (block) old cooling holes. One problem with the brazing approach is that the braze material will incrementally lower the incipient melting temperature of areas of inhomegeneous chemistry in the metal alloy of the component, especially on castings, due to diffusion of boron or silicon into the base metal alloy from the braze material. A second problem with brazing is that wrought alloy properties will be reduced by exposure to brazing procedures. Lower melting brazes could be prone to re-melting with a possibility of re-solidifying elsewhere on metal alloys adversely affected by exposure to the low melting braze constituents. Welding attempts of the old set of cooling holes has proven to introduce substantial distortion into the part associated with solidification of the welds. For both weld and braze repairs, precipitation hardenable alloys such as Iconel 718 or Waspoloy will usually be distorted by the post weld/braze heat treatment required to restore the alloy to a serviceable condition.
To better address the challenges raised by the gas turbine industry to produce reliable and high performance gas turbine engines, an in particular, to provide engines with better designed cooling holes, it is desirable to provide a method for filling cooling holes. Specifically, a method which effectively blocks cooling holes of the component of interest to produce a durable component surface before application of a new TBC is desired. It is also desirable that the materials used to block the cooling holes do not induce any detrimental effects in the substrate.